Self tapping screw for screwing into thermoplastics and the like

ABSTRACT

A retaining screw produced by cold rolling includes a self tapping thread designed to be screwed into thermoplastic material or the like. The screw has a virtually end-to-end cylindrical thread as well as thread teeth, the thread crest of which has the same height on the whole length, while the free space between two teeth remains the same all along the thread The screw presents the following characteristics: either 1) the thread outer diameter Do and the thread inner diameter Dk represent the ratio Q 1  =Do/Dk, in a magnitude of 1.2 to 1.4; 2) the axial distance P between two thread teeth gives with the height H of the latter a ratio Q2=P/H, in a magnitude of 2.75 to 2.9; 3) the point angle of the thread teeth is about 30°; or 1) the thread outer diameter Do and this thread inner diameter gives a ratio Q1=Do/Dk, in a magnitude of 1.25 to 1.65; 2) the axial distance P between two thread teeth gives with the height H of the latter a ratio Q2=P/H, in a magnitude of 2.35 to 2.7; 3) the point angle of the thread teeth is about 30°.

FIELD OF THE INVENTION

The invention relates to a screw, formed by cold rolling, with aself-tapping single-flight thread, for screwing, in particular, intothermoplastics, with an essentially continuously cylindrical threadbottom and with thread teeth which have a vertex of continuously equalheight, the free spaces between adjacent thread teeth being made thesame along the thread.

BACKGROUND AND SUMMARY OF THE INVENTION

A screw of this type is known from German patent specification 39 26000. This screw, regarding which the patent specification expresslypoints out that it is particularly suitable for setting and adjustingdevices, that is to say for so-called movement threads, is to have ahigh-quality thread, by means of which a continuous and exactlymeterable screw-in torque can be achieved. The outlay in terms offorming during the production of the screw by cold rolling is, at thesame time, to be lower than in the case of conventional types of screw.In order to achieve this object, the screw possesses a vertex angle ofabout 40°, thus making it easier for the screw material to flow duringthe cold-rolling process.

The present invention is to provide a screw which, in contrast to theaction of a movement thread, is particularly suitable for screwing, inparticular, into thermoplastics for fastening purposes. When screws ofthis type are screwed into plastic, the latter is displaced by thethread teeth penetrating into the plastic, for which purpose the threadcross section must make sufficient free space available. During thistime, the plastic displaced by the thread teeth is squeezed into thisfree space. In this case, German patent specification 27 54 870, column2, paragraph 1, started from the idea of penetrating as deep as possibleinto the respective plastic workpiece (high bearing depth), for whichpurpose, in the case of a flank angle of about 30°, a correspondinglylarge free space must then be made available. To this effect, in thescrew according to German patent specification 27 54 870, the threadbottom has a contraction, so that a correspondingly large free space isavailable for the plastic displaced by the thread teeth. In this case,the plastic displaced by the thread teeth has to cover, from the regionof the thread tooth which is penetrated into the plastic, into thethread bottom, a distance over which the displaced plastic loses thedirect intimate bond with the non-displaced plastic due to the length ofthis distance, thus making the said displaced plastic less capable ofcontributing to the pull-out force. The term "pull-out force" refers tothe force necessary for tearing out the screwed-in screw. However, alarge free space for receiving displaced plastic results in acorrespondingly small thread-bottom diameter (core diameter), and aconsequence of this may be that, when such a screw is screwed in, inparticular when it is screwed in over a relatively large number ofthread flights, it is overloaded by the torque acting on it and breaksoff.

The idea of displacing the plastic is also of primary importance in thedesign of the screw according to European patent specification0,589,399. This screw, which is designed expressly for a high bearingdepth of the thread flanks, is to achieve a material displacement suchthat the material is compacted on the thread flanks, specifically onthose flanks which, as so-called load flanks, have to absorb tear-outforces acting on the screw. In order to influence the flow of materialin this way, the screw possesses a conical thread bottom, the smallercore diameter of which is adjacent to the said load flank. The flow ofthe plastic is thereby to be directed towards the load flank, where theplastic is then to be compacted. This intended flow of plasticpresupposes that the material is sufficiently heated and converted intoflow and solidification, while the screw is being screwed in. The designof the known screw is therefore based on considerably softening,shifting and compacting the plastic while the screw is being screwed in,as a result of which the plastic is completely released from itsoriginal bond and its structure is changed. As a consequence of this,the load flanks of the screws have to be supported on this materialwhich has been changed in such a way, but precisely this leads to areduction in the tear-out forces of a screw thus screwed in.

The present invention departs from the idea, prevalent hitherto in theprior art, of the thread teeth penetrating as deep as possible into theplastic, which, at first sight, seems plausible in order to achieve hightear-out forces. For this purpose, according to a first variant, thefollowing dimensioning may be used:

1. the outside diameter Do and the core diameter Dk of the thread form aquotient Q1=Do/Dk of a magnitude of 1.2 to 1.4;

2. the axial spacing P of adjacent thread teeth forms, with the height Hof the thread teeth, a quotient Q2=P/H which is between 2.75 and 2.9;

3. the vertex angle of the thread teeth is about 30°.

This dimensioning leads to the screw having relatively low thread teeth,so that the screw sits in the plastic with a relatively low depth ofpenetration. The advantage of this is that the structure of thedisplaced plastic is not impaired to any considerable extent. In orderto achieve high tear-out forces in spite of the relatively low depth ofpenetration, the thread is produced with a relatively low pitch, so thata large number of thread flights engaged with the plastic are provided.

According to a second variant, the following dimensioning may also beused:

1. the outside diameter Do and the core diameter Dk of the thread form aquotient Q1=Do/Dk of a magnitude of 1.25 to 1.65;

2. the axial spacing P of adjacent thread teeth forms, with the height Hof the thread teeth, a quotient Q2=P/H which is between 2.35 and 2.7;

3. the vertex angle of the thread teeth is about 30°.

This latter dimensioning has, in addition, the following effect: thecore diameter possesses a dimension which makes it possible to take intoaccount the forming behaviour of a respective screw blank during itsproduction by rolling, specifically in that the height of the threadteeth, which is building up on the core diameter, is still sufficientlylarge to displace the material of the screw in an advantageous way, inthe region between the thread teeth, during rolling by the pressureexerted on the blank and, consequently, to produce accurately shapedthread teeth which, on the one hand, also have the advantage that theydo not impair the plastic when they penetrate into it, but still have aheight such that they can be shaped with considerable accuracy duringrolling.

So that these effects, made possible by the dimensionings explainedabove, can be fully utilized, the screw according to the inventionpossesses a vertex angle of the thread which is about 30°.

A vertex angle of this kind for screws to be screwed into plastic isknown per se, as shown, for example, by German patent specification 2754 870. The aim, with this screw, is to make a relatively large freespace between the thread flights available for the plastic into whichthe screw is screwed. The tendency followed by this screw amounts,therefore, to displacing as much plastic as possible by means of thethread teeth, that is to say, to working with a high depth ofpenetration, in order thereby to achieve high tear-out forces. Theteachings of this patent specification do not therefore suit the priorart defined by German patent specification 39 26 000, since this priorart, apart from its particular suitability for movement threads, when itis applied to plastic screws, amounts to displacing as little plastic aspossible with these screws.

The combination of the dimensioning features relating to the height andspacing of the thread teeth with the use of a particularly small vertexangle known per se results in a reinforcement of the principle ofdisplacing as little plastic as possible when the screw is screwed intoplastic, since the plastic mass to be displaced on account of the smallvertex angle of 30° is substantially smaller than in the case of avertex angle of 40°, as emerges from German patent specification 39 26000, the result of this being that the interspace existing between thethread flights, this being defined by the pitch of the thread, can bekept shorter, the consequence of this being that the number of threadflights to be anchored in a plastic part is increased correspondingly,if the screw-in length remains the same. This then leads tocorrespondingly increased tear-out forces.

High tear-out forces necessitate a corresponding lcad-bearing capacityof the screws both in the axial and in the tangential direction. Thethinner the cross section of the screw relative to its outside diameter,the greater is the load on the screw, in relation to its cross section,both when it is being screwed in and when it is being tightened. Sothat, in this case, high tear-out forces car be withstood, it hashitherto been necessary for the screw material used for the known screwsto have very high strength, the result of this being that highlytempered materials have had to be used for the screws in question here,these materials, on the one hand, being costly and, on the other hand,reacting sensitively to embrittlement occurring due to hydrogendiffusion. The consequence of this known effect may be that, in theknown firmly tightened screws made of highly tempered material,time-delayed brittle fractures occur after they have been screwed in andtightened, for example by their heads splitting off after a few days.The screw dimensioning according to the invention provides screws havinga relatively large cross section in relation to their outside diameter(as, for example, a look at FIG. 1 clearly shows). The screws accordingto the invention, by virtue of their design over their cross section,can therefore absorb considerably higher torques and axial forces thanhas been possible hitherto. This affords the possibility of producingthe screw according to the invention from a material which has lowerstrength, as compared with conventional screws, that is to say materialscan be used which, because of lower tempering, do not have the tendencyto absorb hydrogen by diffusion and are therefore inclined to brittlefractures.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are illustrated in the figures.

Of these:

FIG. 1 shows a fastening screw screwed into a tube, for fastening aplate according to the first variant,

FIG. 2 shows a detail of the single-flight thread of the screw accordingto FIG. 1 in an enlarged illustration,

FIG. 3 shows a detail of a two-flight thread,

FIG. 4 shows a fastening screw screwed into a tube, for fastening aplate according to the second variant,

FIG. 5 shows a detail of the single-flight thread of the screw accordingto FIG. 1 in an enlarged illustration,

FIG. 6 shows a detail of a two-flight thread.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a screw 1, formed by cold rolling, with the self-tappingthread 2 which extends uniformly over the shank 3. On that side of thescrew 1 which is locates opposite the end of the shank 3, the said screwpossesses the screw head 4 which is provided, here, with a profiledrecess 5 for the insertion of a suitable spanner, by means of which thescrew 1 is screwed into a workpiece, here the tube 6. The screw 1 servesfor fastening the plate 7 to the tube 6. For this purpose, the screw istightened relative to the tube 6 in such a way that its head 4 at thesame time presses the plate 7 against the tube 6.

The particular feature of the fastening screw 1 according to theinvention can be seen clearly from FIG. 1, specifically its relativelylow depth of penetration into the material of the tube 6 and, ascompared with this, the relatively large core diameter Dk of the thread2, the continuously cylindrical thread bottom 8 of which leaves only arelatively small free space relative to the inner face 9 of the tube 6.

FIG. 2 illustrates a detail of the shank 3 according to FIG. 1 in anenlarged illustration. This detail illustrates the core diameter Dk, theoutside diameter Do and the axial spacing P and the height H of thethread teeth 10. Moreover, the flank angle of the thread teeth isspecified as 30° in FIG. 2.

It may be gathered from FIG. 2 that, in the screw, illustrated enlarged,the quotient Q1=Do/Dk is 1.3. here [sic], the quotient Q2=axial spacingP/height H has the value 2.85.

FIG. 3 illustrates a detail of a two-flight thread 11 which, apart fromthe number of flights, is otherwise largely identical to the thread 2according to FIG. 2. The axial spacing P in the thread 11 according toFIG. 3 is measured, here, between the two adjacent thread teeth of theone thread flight and of the other. This results, as in the exemplaryembodiment according to FIG. 1, in the quotient Q2=axial spacingP/height H with a value of 2.85.

The fastening screws illustrated in FIGS. 1 and 2 are those which areusually produced with an outside diameter of 1 mm to 10 mm. The screwaccording to the invention is suitable for screwing not only into thecurrently customary thermoplastics, but also into all other materialswhich have properties similar to those of thermoplastics.

FIG. 4 shows a screw 21, formed by cold rolling, with the self-tappingthread 22 which extends uniformly over the shank 23. On that side of thescrew 21 which is located opposite the end of the shank 23, the saidscrew possesses the screw head 24 which is provided, here, with aprofiled recess 25 for the insertion of a suitable spanner, by means ofwhich the screw 21 is screwed into a workpiece, here the tube 26. Thescrew 21 serves for fastening the plate 27 to the tube 26. For thispurpose, the screw is tightened relative to the tube 26 in such a waythat its head 24 at the same time presses the plate 27 against the tube26.

The particular feature of the fastening screw 21 according to theinvention can be seen clearly from FIG. 4, specifically its relativelylow depth of penetration into the material of the tube 26 and, ascompared with this, the relatively large core diameter Dk of the thread22, the continuously cylindrical thread bottom 28 of which leaves only arelatively small free space relative to the inner face 29 of the tube26.

FIG. 5 illustrates a detail of the shank 23 according to FIG. 4 in anenlarged illustration. This detail illustrates the core diameter Dk, theoutside diameter Do and the axial spacing 4 and the height H of thethread teeth 30. Moreover, the flank angle of the thread teeth isspecified as 30° in FIG. 5.

It may be gathered from FIG. 5 that, in the screw, illustrated enlarged,the quotient Q1=Do/Dk is 1.46. Here, the quotient Q2=axial spacingP/height H has the value 2.48.

FIG. 6 illustrates a detail of a two-flight thread 31 which, apart fromthe number of flights, is otherwise largely identical to the thread 22according to FIG. 5. The axial spacing P in the thread 31 according toFIG. 6 is measured, here, between the two adjacent thread teeth of theone thread flight and of the other. This results, as in the exemplaryembodiment according to FIG. 4, in the quotient Q2=axial spacingP/height H with a value of 2.48.

The fastening screws illustrated in FIGS. 4 and 5 are those which areusually produced with an outside diameter of 1 mm to 10 mm. The screwaccording to the invention is suitable for screwing not only into thecurrently customary thermoplastics, but also into all other materialswhich have properties similar to those of thermoplastics.

For soft plastics, a screw according to the invention is particularlysuitable, in which, within the latitude of variation of the flank angleof approximately 30°, a value is selected which leads to particularlypointed thread flights, specifically with a flank angle of approximately25°.

What is claimed is:
 1. Fastening screw (1), formed by cold rolling, witha self-tapping thread (2, 11) for screwing into thermoplastics, andmaterials which have properties similar to those of thermoplastics withan essentially continuously cylindrical thread bottom (8) and withthread teeth (10) which have a vertex of continuously equal height, thefree spaces between adjacent thread teeth (10) being made the same alongthe thread (2, 11), characterized by the combination of the followingfeatures,1. the outside diameter Do and the core diameter Dk of thethread form a quotient Q1=Do/Dk of a magnitude of 1.2 to 1.4;
 2. theaxial spacing P of adjacent thread teeth forms, with the height H of thethread teeth, a quotient Q2=P/H which is between 2.75 and 2.9; and 3.the vertex angle of the thread teeth is about 30°.
 2. Fastening screw(21) formed by cold rolling with a self-tapping thread for screwing intothermoplastics and materials which have properties similar to those ofthermoplastics with an essentially continuously cylindrical threadbottom and with thread teeth which have a vertex of continuously equalheight, the free spaces between adjacent thread teeth being made thesame along the thread, characterized by the combination of the followingfeatures,1. the outside diameter Do and the core Diameter Dk of thethread form a quotient Q1=Do/Dk of a magnitude of 1.25 to 1.46;
 2. theaxial spacing P of adjacent thread teeth forms, with the height H of thethread teeth, a quotient Q2=P/H which is between 2.35 and 2.7; and 3.the vertex angle of the thread teeth is about 30°.